This invention relates to improved methods and apparatus for plasma processing.
Plasmas are used in a wide variety of applications for carrying out process operations. Exemplary of such process operations is the use of RF induction power to produce non-thermal plasmas, also referred to as non-equilibrium plasmas. The manufacture of semiconductor devices is one area in which non-thermal plasmas are used extensively. The non-thermal plasmas are used for etch processes wherein the non-thermal plasmas are used to generate reactive species in a gas to accelerate reactions between the species and a solid surface. The etch process can be a general removal of components on the surface as in a cleaning process or the selective removal of material from certain areas on the surface through use of a masking material that has been previously patterned. Non-thermal plasmas are used to promote deposition reactions wherein gas phase species are caused to react to form a solid product that deposits on surfaces. During the manufacture of semiconductor devices, etch processes involving RF plasmas and deposition processes involving RF plasmas are used repeatedly during the fabrication process. One of the main benefits of using the non-thermal plasma is the ability of the non-thermal plasma to stimulate chemical reactions that would otherwise require temperatures that are too high for use in the fabrication of semiconductor devices.
RF power non-thermal plasmas are also used as cleaning processes in manufacture of semiconductor devices. The non-thermal plasmas are commonly used to strip photoresist materials from semiconductor wafers as part of post etch wafer clean procedures. The photoresist material serves as a mask material during etch processes used in patterning the surface of the devices. Resist material is stripped from the surface of the wafers by creating a non-thermal plasma in a gas containing oxidizing species such as oxygen and possibly halogen species that are capable of reacting with and volatilizing the resist material. In some applications, the non-thermal plasma is maintained at a position upstream of the wafer. Reactive species generated in the non-thermal plasma flow downstream and react with the wafer surface for the stripping process.
Another cleaning process that uses non-thermal plasmas is the cleaning of reaction chambers used in semiconductor manufacturing. Sometimes, the reaction chambers used in plasma etch processes experience a buildup of deposits from the etch process. These deposits need to be removed as part of the reactor maintenance process. In addition, the reactors that are used in deposition processes for semiconductor device fabrication undergo a buildup of deposits on the reactor walls; the wall deposit must be removed as part of reactor maintenance. Non-thermal plasmas generated using RF power and gases containing species that are reactive with the deposits have been used to volatilize and removed the deposits built up on the walls of etch reactors and deposition reactors.
RF power plasmas have also been used for decomposition of chemical compounds that are hazardous or otherwise undesirable. Some of the compounds are highly refractory in nature and are difficult to decompose. Examples of compounds that have been decomposed or abated with plasmas include chlorofluorocarbons (CFC) and perfluorocompounds (PFC).
For some applications, there are major problems with the methods and apparatus used for carrying out plasma processes. One set of problems is related to incompatibilities between design requirements for the apparatus and operating requirements for the apparatus. For example, depending on the type of plasma application, the materials used in the apparatus may have extreme requirements in terms of chemical, mechanical, electrical, and thermal properties. The problem of meeting the materials requirements is further aggravated by the need to use materials that are preferably readily available and economical. Still further difficulties are imposed by the preference to operate the apparatus using commonly available industrial facilities and the desire for equipment that is simple in operation and reliable under a variety of conditions.
One frequently encountered problem is the need to remove heat generated by the plasma. This is a particularly challenging problem when the materials exposed to the plasma are dielectric materials. Typically, the dielectric materials used in plasma equipment are selected because the materials are substantially transparent to electromagnetic radiation such as RF power and/or because of the chemical or thermal stability of the material. The dielectric materials tend to have poor thermal conductivity. Furthermore, commonly used methods for cooling the dielectric materials frequently employ materials, such as metals, that have very high thermal conductivity compared to that for dielectrics. The differences in the thermal conductivity for dielectric and thermal conductivity for cooling materials can produce large thermal gradients in the dielectric material. The thermal gradients in the dielectric material along with the thermal expansion properties of the dielectric can produce physical stresses in the dielectric that can cause failures such as breakage of the dielectric. These failures can cause a loss in productivity and an increase in operating costs for plasma processing.
In addition to the need for moderating the temperature gradients in the dielectric material, there is also a need for maintaining the temperatures of the materials exposed to the plasma at suitable temperatures to prevent corrosion. This is particularly important when the plasma process uses reactive or corrosive gases. Specifically, the heat produced by the plasma must be removed and the surfaces exposed to the plasma must be maintained at temperatures at which corrosion is not a problem.
An additional problem for plasma processing occurs when changes in the plasma processing conditions cause the plasma to change its distribution in the plasma chamber. These changes can rapidly alter the heat flux from the plasma and produce new temperature gradients in dielectric materials exposed to the plasma. The rapid changes in temperature gradients can contribute to stresses that can cause the dielectric materials to break.
Some of the problems related to heat removal for plasma processing could be alleviated by using additional temperature controllers, additional temperature control systems, non-standard materials, and non-standard process facilities for process cooling. However, those choices lead to greater complexity for the system, greater initial cost, and greater operating cost.
Clearly, there are numerous applications requiring plasma processing systems. Unfortunately, typical methods and apparatus for old-style plasma processing systems have characteristics that may be undesirable for some applications. There is a need for plasma processing methods and apparatus that are simple in operation, use standard industrial facilities, provide high reliability, and have the versatility to handle a wide range of plasma conditions.